As a dust collecting filter typified by a diesel particulate filter (hereinafter sometimes referred to as the “DPF”), a filter having a honeycomb structure and made of a ceramic (a honeycomb filter) is used.
Usually as the filter for use in such an object, as shown in FIGS. 14 and 15, there is broadly used a wall flow type honeycomb filter 51 having a honeycomb structure 52 in which a plurality of cells 59 communicating between two end faces and constituting fluid passages are partitioned by porous partition walls 57, and plugging portions 58 arranged in checkered patterns complementarily formed by one end face and the other end face of the honeycomb structure 52 so that one of two opening end portions of each cell 59 is plugged (for example, see Patent Document 1).
When an exhaust gas containing particulate matter (hereinafter sometimes referred to as the “PM”) is supplied from one end face 53 of this honeycomb filter 51, this exhaust gas flows into the honeycomb filter from the one end face 53 thereof, and the PM contained in the exhaust gas is removed. Afterward, the gas flows out of the other end face 55. Specifically, first the exhaust gas flows into cells 59b having end portions which are not plugged in the one end face 53 and having end portions plugged in the other end face 55. The gas passes through the porous partition walls 57, roves to cells 59a having end portions plugged in the one end face 53 and having end portions which are not plugged in the other end face 55, and is discharged from these cells 59a. Then, in this case, the partition walls 57 function as a filter layer, and the PM in the gas is trapped by the partition walls 57 and accumulated on the partition walls 57.
In such a honeycomb filter, the PM is accumulated on the partition walls of the honeycomb structure constituting the filter, whereby a pressure loss rises. Therefore, in such a honeycomb filter, when a predetermined amount of the PM is accumulated on the partition walls, the honeycomb filter is forcibly heated to a high temperature, and the PM accumulated on the partition walls is oxidized and removed.
[Patent Document 1] JP-A-2001-269585
However, during the oxidation and removal of the PM, a fuel is excessively jetted to burn the PM, so that there is a problem that a fuel consumption ratio becomes unfavorable and an economical property deteriorates. Moreover, the temperature of the honeycomb filter becomes high, so that there is another problem that the honeycomb filter is damaged.
Moreover, to reduce the PM accumulation, it is suggested that an oxidation catalyst for oxidizing the PM be carried on the porous partition walls. However, in a case where the amount of PM is large, there are problems that the PM is accumulated in layers on the partition walls and that a frequency at which the oxidation catalyst cones in contact with the PM decreases, so that a sufficient effect cannot be obtained.
Furthermore, in a honeycomb filter, it is also suggested that surfaces of fluid inflow side of the partition walls, through which a fluid passes, be constituted of a porous body having large pore diameters, and surfaces of a side opposite to the fluid inflow side of the partition walls (surfaces of a fluid outflow side of the partition walls) be constituted of a porous body having small pore diameters. However, such a honeycomb filter has a problem that the pressure loss rapidly rises when the PM is accumulated on the partition walls.
In addition, to simply reduce the rise of the pressure loss, for example, it is considered that the partition walls be formed only of the porous body having large pore diameters, but there are problems that such a honeycomb filter has a low trapping efficiency, and cannot be used as the above-mentioned DPF or the like.